Catalytic cracking is a petroleum refining process that is applied commercially on a very large scale. A majority of the refinery petroleum products are produced using the fluid catalytic cracking (FCC) process. An FCC process typically involves the cracking of heavy hydrocarbon feedstocks to lighter products by contacting the feedstock in a cyclic catalyst recirculation cracking process with a circulating fluidizable catalytic cracking catalyst inventory consisting of particles having a mean particle size ranging from about 20 to about 150 μm, preferably from about 50 to about 100 μm.
The catalytic cracking occurs when relatively high molecular weight hydrocarbon feedstocks are converted into lighter products by reactions taking place at elevated temperature in the presence of a catalyst, with the majority of the conversion or cracking occurring in the vapor phase. The feedstock is converted into gasoline, distillate and other liquid cracking products as well as lighter gaseous cracking products of four or less carbon atoms per molecule. The gas partly consists of olefins and partly of saturated hydrocarbons. Bottoms and coke are also produced. The cracking catalysts typically are prepared from a number of components, each of which is designed to enhance the overall performance of the catalyst. FCC catalysts are generally composed of zeolite, active matrix, clay and binder with all of the components incorporated into a single particle.
Alumina is an inorganic oxide based active matrix used in FCC catalysts. See U.S. Pat. Nos. 4,086,187; 4,206,085; and 4,308,129. Alumina hydrate is typically used for this purpose, with boehmite or microcrystalline boehmite, also called pseudoboehmite, frequently used. The alumina can be further treated with acid to improve the properties of resulting alumina matrix once the final catalyst is formed. The acid is added in concentration up to 2 moles of acid equivalence per mole of Al2O3, primarily to improve attrition resistance. Treating alumina with acid in this fashion is also commonly known as “peptizing”.
Other components utilized in FCC catalysts prepared from peptized aluminas can include rare earth such as lanthanum or cerium. See the '187 and '085 patents above. It is also taught, however, that such catalysts should be substantially free of rare earth metals, and other elements such as yttrium. See the aforementioned '129 patent, and in particular Column 4, lines 11-22 thereof.
It is believed however that adding zeolites to an acidified alumina leads to degradation of the zeolite structure during manufacture of the catalyst and during use in a FCC process. In particular it is believed that the lower pH in catalyst preparation slurries containing peptized aluminas lead to leaching of alumina from the silica alumina structures of the zeolite, thereby leading to collapse of the zeolite structure and loss of surface area. Loss of surface leads to loss in cracking activity, and therefore requiring more frequent replacement of catalyst inventory.